The present invention is directed to a method for measuring the optical attenuation of an optical medium, whereby an optical measuring signal of a first measuring transmitter or means is coupled into the optical medium and is then coupled out therefrom after traversing the medium and is measured with a first measuring receiver in a first measuring process.
Copending U.S. Patent Application Ser. No. 06/755,276, filed July 15, 1985, claims priority from German Published Application No. 34 29 947 and discloses a measuring device wherein light is coupled from a measuring transmitter into the light waveguide proceeding through a splice location. The light waveguide is thereby guided in a precise, defined fashion in the coupling region so that a largely reproducible coupling relationship can be obtained. In detail, the light to be coupled in is thereby supplied to the actual coupling region by a second light waveguide, whereby a particularly defined and clean in-coupling is achieved.
The optical attenuation of the optical medium, for example of a splice location, of a coupler, of a light waveguide cable or the like, represents an important measured quantity. For example, the optical attenuation, thus, considerably limits the usable product of band width and length of an optical transmission link. Global attenuation measurements, such as, for example the overall attenuation of the optical transmission length, and the attenuation of a sub-link are required. Thus, local attenuation discontinuities can also be added thereto, such as, for example, as occur at an fiber splice, couplers or at other optical components.
Particularly for the employment in optical fibers or, respectively, light waveguide cables, what is referred to as a time-domain reflectometry method and the transmitted light measuring method are known methods for measuring the optical attenuation. The advantages of an optical time-domain reflectometry (OTDR) are that only one end of the light waveguide need be accessible for the measurement. However, a disadvantage of the optical time-domain reflectometry method is the quickly decreasing measuring precision which will occur with increasing distance from the in-coupling location. In the transmitted light method, both ends of the light waveguide must be simultaneously accessible; however, a higher measuring precision, when compared to an optical time-domain reflectometry, will occur. Thus, this measuring method is utilized in the laboratories and in quality protection programs. The measurement with the transmitted light method requires a reference measurement that is acquired in a known fashion by cutting the specimen of the light waveguide back. In addition to the fact that this method is time-consuming and does not work destruction-free, the repeated coupling of the cut-back light waveguide to the respective receiver also limits the obtainable measuring precision.
The measurement of local attenuation discontinuity, for example at a splice location, is generally executed with the assistance of an optical time-domain reflectometry. In practice, however, it must be assumed that the precise measurement of the attenuation discontinuity requires an application of the optical time-domain reflectometry at each of the two light waveguide ends because of the unavoidable tolerances in the radial refractive index geometry of the fibers of the optical light waveguide fibers and because of the unsteadiness of the field diameter proceeding therefrom at the fiber joint. A further disadvantage is also that the measuring precision is dependent on the distance between the joint and the in-coupling location of the measuring signal. Measurements of, for example, the splice attenuation with the transmitted light method is, however, without practical significance because of the reference measurement that is required.
In the measuring method of the above-mentioned U.S. Patent Application, only the out-coupled power following, for example, the splice location is not measured and the quantity of the in-coupled power at the transmission side, however, is not measured. A corresponding measuring device for the thermic splicing enables an estimate of the splice attenuation with relatively precise values when identical light waveguides are employed and given the assumption of a coupling optic having ideal fiber end faces and an ideal arrangement of fiber cores. The splice attenuation .alpha. is defined by the measuring of the power modification .DELTA..alpha. proceeding and following the production of the splice connection upon additive consideration of the mean air splice attenuation of the ideal coupling optics. The following relationship thereby applies: EQU .alpha.=mean air splice attenuation+.DELTA..alpha. (dB).
In practice, the necessary assumptions cited above are not always met with the high precision that is required, as a result whereof the adding of a constant, means air splice attenuation can lead to faulty values. Intrinsic losses, which occur given the mismatching of non-identical light waveguides, are thereby not measurable. Added thereto is that the attenuation of fiber splices that have already been produced, i.e., of a finished fiber splice, cannot be identified with such a method.